Process for production of cellulose nanofibers from miscanthus x. giganteus and composites therefrom

ABSTRACT

A process for the isolation of cellulose nanofibers, in particular carboxylic acid functionalized cellulose nanofibers (MxG-CNF-CO2H) from the sustainable grass hybrid  Miscanthus  x.  giganteus  (MxG). The process includes the steps of performing bleach treatment on a quantity of MxG followed by ammonium persulfate oxidation. The resulting MxG-CNF-CO2Hs have good dispersibility in aqueous and polar organic solvents. They are also able to form gels at relatively low concentrations. The MxG-CNF-CO2H show significantly higher reinforcement effect when compared to carboxylic acid functionalized cellulose nanocrystals at the same weight percent in a composite composition.

FIELD OF THE INVENTION

The present invention relates to a process for the isolation of cellulose nanofibers, in particular carboxylic acid functionalized cellulose nanofibers (MxG-CNF-CO₂H) from the sustainable grass hybrid Miscanthus x. giganteus (MxG). The process includes the steps of performing bleach treatment on a quantity of MxG followed by ammonium persulfate oxidation. The resulting MxG-CNF-CO₂Hs have good dispersibility in aqueous and polar organic solvents. They are also able to form gels at relatively low concentrations. The MxG-CNF-CO₂H show significantly higher reinforcement effect when compared to carboxylic acid functionalized cellulose nanocrystals obtained from the same biosource at the same weight percent in a composite composition.

BACKGROUND OF THE INVENTION

Given that the production of cellulose in the biosphere is ca. 7.5×10¹⁰ tons per year,¹ it can be considered significant source of sustainable raw materials. As such, the increasing demand for environmentally-friendly products has led to a resurgence in the interest of cellulose-based materials.² Cellulose is a carbohydrate polymer consisting of β-D-anhydroglucose repeat units and exists as a hierarchical structure of semi-crystalline cellulose microfibrils, each of which has 30-40 cellulose chains and a cross-sectional width of ca. 3-4 nm, depending on its origin.³⁻⁴ The growing demand for bio-renewable and green nanomaterials has led to an increasing amount of attention being paid to nanocelluloses in the last decade or so.^(3, 5) This generally involves breaking down the natural cellulose fibers into nanosized structures using mechanical and/or chemical processing in order to overcome the strong intra- and intermolecular hydrogen bonds of the numerous hydroxyl groups on the cellulose chains.⁶⁻⁷

Two major classes of nanocellulose, cellulose nanocrystals (CNC) and cellulose nanofibers (CNF), have been prepared from a wide variety of biosources by various chemical and mechanical treatments.^(4, 8-10) CNCs are rod-like highly crystalline nanoparticles, while CNF have a much higher aspect ratio and consist of both crystalline and non-crystalline domains. CNCs are usually prepared by hydrolysis of natural cellulose using acids, such as hydrochloric acid,¹¹⁻¹² hydrobromic acid,¹³⁻¹⁴ sulfuric acid,¹⁵⁻¹⁶ and phosphoric acid.¹⁷⁻¹⁸ During hydrolysis, the non-crystalline regions of cellulose are preferentially hydrolyzed and removed while the crystalline regions remain intact and thus yield the highly crystalline, rod-like cellulose nanocrystals. CNCs functionalized with charged groups allow for easier dispersion in solvents and more facile processing. Sulfate and phosphate functionalized CNCs are obtained directly upon hydrolysis with sulfuric acid¹⁵⁻¹⁶ and phosphoric acid,¹⁷⁻¹⁸ respectively. CNCs isolated with HCl are uncharged and are usually converted into carboxylic acid functionalized CNCs via a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl)-mediated oxidation. CNCs have been prepared from several sources of cellulose, such as bleached softwood pulp,¹⁹ bleached hardwood pulp,¹⁶ cotton,²⁰ tunicates,²¹⁻²² bacterial cellulose,²³⁻²⁴ ramie,²⁵ and sisal.²⁶ The lateral dimension of CNCs is between 3 and 20 nm, and the length is 100-1000 nm depending on the original biosource and isolation procedure⁴ The concentration of acid employed during isolation as well as the ratio of acid to cellulose, reaction temperature and reaction time are all important parameters that impact the nature, yield and stability of the CNC isolated from the biosource. More recently, alternative isolation procedures, such as ammonium persulfate oxidation²⁷ and sodium periodate-chlorite sequential oxidation,²⁸ have also be developed. Both these processes produce CNCs functionalized with surface carboxylic acid groups directly from the biosource investigated.

The conversion of cellulose fibers into CNFs by pure mechanical treatment requires significant energy input, and usually results in materials that still contain numerous bundles of the microfibers.²⁹ In order to reduce the energy consumption during the mechanical processing; as well as improve the efficiency of the conversion of cellulose fibers into nanosized structures, chemical or enzymatic pretreatments are generally applied on the cellulose prior to mechanical treatment. One of the major chemical pretreatments is TEMPO-mediated oxidation,³⁰⁻³¹ which selectively oxidizes the primary alcohol groups of cellulose into carboxylate moieties. The addition of these negative charges facilitates the generation of CNFs upon application of mechanical (e.g., ultrasonic) treatment on account of electrostatic repulsion. Introducing carboxylate groups onto cellulose fibers by sequential periodate-chlorite oxidation is another efficient chemical pretreatment, which also increases electrostatic repulsion between the fibrillar constituents and results in an easier nanofibrillation with the use of a homogenizer.³²⁻³³ The dimensions of CNFs produced by these procedures is generally between 3 and 20 nm in width, with lengths from hundreds of nanometers to a few micrometers depending on the biosource and the protocol employed. CNFs prepared from TEMPO-mediated oxidation have been widely used as reinforcement fillers in polymer composites. Addition of CNFs into a polymer matrix yields polymer composites that exhibit enhanced storage modulus ascribed to not only the formation of interfacial hydrogen bonding interactions between the CNFs and the polymer matrix, but also the formation of hydrogen bonded percolating CNF networks. A wide range of polymer:CNF composites, such as epoxy nanocomposites,³⁴ polyvinyl alcohol (PVA) films,³⁵ starch-based biodegradable resin,³⁶ polylactic acid) (PLA) films,³⁷ and thermoplastic elastomers,³⁸ have been prepared and investigated.

Ammonium persulfate (APS) is a low cost oxidant that has been shown to have low long-term toxicity.²⁷ As mentioned above it has been used to isolate CNCs directly from certain biosources, such as flax, wood and cotton. The resulting CNCs are carboxylate functionalized presumably on account of the oxidation of some of the surface primary hydroxyl groups during the isolation procedure.^(27, 39-40) Two main byproducts are generated during this oxidation process, ammonium sulfate and H₂SO₄, with the latter accounting for ca. 60% of the total sulfate ions in solution.⁴¹ CNCs obtained from APS oxidation of sugarcane bagasse pulp showed a higher crystallinity and better thermal stability compared to carboxylated CNCs prepared via HCl hydrolysis and TEMPO-mediated oxidation, albeit in lower yields.⁴² Polarized light microscopy showed the suspensions of CNCs prepared by APS oxidation of cotton or wood pulp can also form chiral nematic liquid crystalline phases above a critical concentration.⁴³ CNCs from APS oxidation of cotton also showed better water resistance and oxygen barrier properties for improving the performance of food-packaging materials.⁴⁴ Overall, the APS oxidation is a simple, relatively mild procedure which can be used to directly obtain carboxylated CNCs. However, to the best of our knowledge, no research has been reported on its use to access cellulose nanofibers from a biosource.

MxG is a perennial grass hybrid and has a composition of about 50% cellulose, 25% hemicellulose and 12% lignin.⁴⁵ MxG is currently farmed in Europe and the United States primarily for combustion to generate electricity and heat or for the preparation of biofuels.⁴⁶ MxG can be planted in poor quality soil, requires little herbicide, fertilizer or water and can be harvested in high yield. It is fast growing, up to 12 feet tall in a two-year period, reaching its peak dry biomass production within 3 years after planting and can be harvested for 15-20 years without replanting. Moreover, it is a rhizomatous 04 grass species that has a high carbon dioxide fixation rate, thus it has a significantly higher harvestable biomass compared to other plants, producing annual biomass yields of 20-26 tons dry weight per hectare.⁴⁷⁻⁴⁸ All these advantages make MxG a highly sustainable source for the production of nanocellulose. Recently, we reported protocols for accessing sulfate or carboxylic acid functionalized cellulose nanocrystals (CNCs) using H₂SO₄ hydrolysis, or HCl hydrolysis followed by TEMPO oxidation, on alkaline and bleach treated MxG.⁴⁹ The ribbon-like MxG-CNCs show a high crystallinity (>85%), high aspect ratio (60-70) and good polymer reinforcing capability.

SUMMARY OF THE INVENTION

In view of the above, science and industry still need cellulose nanomaterials, in particular nanofibers, that can be produced relatively inexpensively and with a reliable, uncomplicated process. The art also needs nanocellulose materials that exhibit desirable dispersibility preferably in aqueous and polar organic solvents, and exhibit good reinforcement capabilities, particularly in polymers.

The problems noted above and others are solved by methods and nanofibers of the present invention.

Therefore, in one embodiment of the present invention, methods for preparing carboxylic acid functionalized cellulose nanofibers are disclosed, including the steps of performing a bleach treatment on a quantity of MxG followed by performing ammonium persulfate oxidation on the solids recovered after the bleach treatment.

In a further embodiment, the present invention provides MxG-CNF-CO₂Hs having relatively high aspect ratios as determined by atomic force microscopy analysis.

Still another embodiment provides MxG-CNF-CO₂Hs that exhibit desirable thermostability as measured by thermal gravimetric analysis.

Yet another embodiment of the invention provides MxG-CNF-CO₂Hs readily dispersible in aqueous and polar organic solvents.

In still further embodiments of the present invention, polymer composites containing MxG-CNF-CO₂Hs are provided, with the latter enhancing mechanical properties of the polymer.

In one aspect, a process for preparing cellulose nanofibers from MxG is disclosed, comprising the steps of performing a bleaching step on a quantity of the MxG; and thereafter subjecting a solid material obtained from the bleaching step to ammonium persulfate oxidation and obtaining carboxylic acid functionalized cellulose nanofibers, MxG-CNF-CO₂Hs.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood and other features and advantages will become apparent by reading the detailed description of the invention, taken together with the drawings, wherein:

FIG. 1 illustrates (a) stalks of MxG, (b) ground-up MxG powder, (c) after bleach treatment, and (d) freeze dried MxG nanofibers after APS oxidation and sonication;

FIG. 2 illustrates SEM images of (a) MxG powder, (b) after bleach treatment, (c) after APS oxidation, and (d) after sonication;

FIG. 3 illustrates an AFM image of MxG-CNF-CO₂H;

FIG. 4 illustrates FTIR spectra of the ground MxG powder, MxG powder after bleaching, MxG-CNF-CO₂H in the basic/sodium and acidic/protonated form and MxG-CNC-CO₂H;

FIG. 5 illustrates X-ray diffraction patterns of ground MxG powder, bleached MxG, MxG-CNF-CO₂H and MxG-CNC-CO₂H;

FIG. 6 illustrates (a) Weight loss and (b) derivative weight loss of MxG powder, bleached MxG, MxG-CNC-CO₂H and MxG-CNF-CO₂H;

FIG. 7 shows pictures of water suspensions of (a) MxG-CNC-CO₂Hs, (b) MxG-CNF-CO₂Hs, (c) MxG-CNC-CO₂Hs after one day standing, and (d) MxG-CNF-CO₂Hs after one day standing at concentrations of (i) 0.2 wt. % (ii) 0.4 wt. % (iii) 0.6 wt. % (iv) 0.8 wt. % and (v) 1.0 wt. %;

FIG. 8 shows pictures of DMF suspensions of (a) MxG-CNC-CO₂Hs, (b) MxG-CNF-CO₂Hs, (c) MxG-CNC-CO₂Hs after one day standing, and (d) MxG-CNF-CO₂Hs after one day standing at concentrations of (i) 0.2 wt. % (ii) 0.4 wt. % (iii) 0.6 wt. % (iv) 0.8 wt. % and (v) 1.0 wt. %;

FIG. 9 illustrates (a) DMA temperature sweeps of MxG-CNF-CO₂H/PVAc and MxG-CNC-CO₂H/PVAc nanocomposites. (b) Tensile storage modulus of MxG-CNF-CO₂H/PVAc and MxG-CNC-CO₂H/PVAc nanocomposites at 80° C.;

FIG. 10 illustrates an AFM image of MxG-CNFs obtained directly from the treatment of MxG ground stalk with APS;

FIG. 11 illustrates histograms for (a) length and (b) width distribution of MxG-CNF-CO₂H from the bleach treatment followed by APS oxidation;

FIG. 12 illustrates FTIR spectra of the starting ground MxG powder, bleached MxG powder, MxG-CNF-CO₂Na (sodium form), MxG-CNF-CO₂H (acidic form) (MxG-CNF-CO₂H in their acidic form were prepared by suspending the freeze-dried MxG-CNF in 0.2 M HCl overnight, followed by washing/centrifugation with deionized water) and MxG-CNC-CO₂H; and

FIG. 13 illustrates storage (G′) and loss (G″) modulus as a function of oscillatory strain for (a) MxG-CNF-CO₂H and (b) MxG-CNC-CO₂H suspensions at different concentrations (0.2-1.0 wt. %).

DETAILED DESCRIPTION OF THE INVENTION

Carboxylic acid functionalized cellulose nanofibers derived from MxG (MxG-CNF-CO₂Hs) are disclosed herein. The cellulose nanofibers can be prepared by a facile two-step process that comprises the steps of bleach treatment followed by ammonium persulfate oxidation. The MxG-CNF-CO₂Hs can be utilized in a wide range of compositions and have particular application as a nanofiller, preferably in polymer compositions.

A desired quantity of MxG is obtained and the stalks thereof are processed to reduce the average particle size of the solid material to a smaller average particle size, for example by grinding, milling, crushing or the like. Any suitable equipment known in the art can be utilized, for example, but not unlimited to mills, blenders and crushers, for example various ball mills, hammer mills, roller mills, presses, vibration mills, jet mills, cone crushers, hammer crushers and jaw crushers.

In one embodiment, the reduced size MxG has an average particle size in the range of about 300 to about 500 μm. MxG of smaller average particle sizes can be utilized as well in other embodiments.

Bleaching Process

After MxG is processed to a desired consistency or particle size or otherwise obtained in a suitable form, a bleaching step is performed. The MxG of desired consistency is soaked, immersed, or otherwise wetted or contacted with a bleaching solution for a sufficient amount of time to convert an amount of lignin present in the material into a soluble phenolic compound.

In various embodiments, the bleaching step utilizes an oxidizing agent, for example chlorine-containing compounds such as sodium chlorite (NaClO₂), sodium hypochlorite and/or calcium hypochlorite. Hydrogen peroxide can also be utilized. The oxidizing agent utilized is preferably stable at high pH and is activated with an acid or acid liberating agent to bring down the pH when bleaching takes place. The bleaching species in sodium chlorite is 0102 gas which is liberated below a pH of 6. In certain embodiments, concentration of the oxidizing agent is about 2% wt. in the bleaching solution.

The bleaching step is performed under acidic conditions such as a pH between about 4 and about 6. pH is adjusted by utilizing an acid, such as glacial acidic acid or other acid liberating agent. A suitable amount of MxG is added to the bleaching solution in order to achieve the desired result. That said, in some embodiments the weight ratio of MxG to solution is about 1:20. In various embodiments, the bleaching solution also includes an acid, such as glacial acidic acid.

In a preferred embodiment, the bleaching solution including the MxG material is mixed, preferably at an elevated temperature such as between 60° C. and 80° C., for a desired time period, such as between 40 minutes and 120 minutes preferably sufficient to convert lignins present in the material as described hereinabove. One of ordinary skill in the art will recognize that processing time will depend upon many factors such as concentration of bleaching agents utilized, processing temperature, etc.

After the bleaching step has been performed to a desired degree, the MxG-containing solution is filtered, and preferably washed with deionized water. The resulting slurry is preferably dried before being subjected to ammonium persulfate oxidation, Drying can take place in any suitable vessel, for example an oven. In various embodiments, the drying can take place at temperatures ranging from about 40 to about 120° C. and preferably about 40° C. Drying generally results in a powder-like composition. Drying temperatures should be low enough in order to prevent degradation of the bleached MxG.

Ammonium Persulfate Oxidation

In a further step of the invention an ammonium persulfate solution is prepared such that APS is present in a concentration from about 0.5 M to about 2.0M. The bleached MxG material, obtained through the bleaching step, is similarly soaked, immersed or otherwise wetted or contacted with the desired APS solution.

Preferably the MxG APS solution is mixed at a desired temperature for a suitable period of time in order to sufficiently oxidize the material to form the MxG cellulose nanofibers. That said, in various embodiments, processing temperature ranges generally from about 50° C. to about 90° C., and preferably is about 90° C.

The oxidation process can be performed for a suitable period of time, such as about 8 hours to about 24 hours and preferably for about 16 hours in an embodiment where 1M APS solution is used. Reaction times will obviously vary depending upon reaction conditions as known to those of ordinary skill in the art.

After APS oxidation has been performed, the oxidized mixture is preferably cooled to room temperature and centrifuged, such as at 8,000 rpm for eight minutes in various embodiments. The centrifugation step is repeated if desired, for example with deionized water and sodium hydroxide, such as 1M sodium hydroxide until the pH of the suspension reaches about 8, before being dialyzed with deionized water for a period of time, such as two days.

In various embodiments, the oxidized MxG-CNF-CO₂Hs are optionally subjected to further processing and size reduction, for example utilizing an ultrasonic homogenizer, for example at 20 kHz and 200 W output power for 2 minutes with a 13 mm probe tip diameter, utilizing a Branson SFX550. Large agglomerates can be removed from suspension by centrifugation, for example at 8,000 rpm for a period of time such as 5 min. The resulting supernatant can then be dried as desired. Lyophilization or freeze drying is utilized in preferred embodiments. The MxG-CNF-CO₂Hs generally appear as a white powder and can be utilized as desired.

MxG-CNF-CO₂H Properties

MxG-CNF-CO₂Hs prepared according to the procedures described herein exhibit many desired characteristics. In one embodiment, MxG-CNF-CO₂H have from about 1000 to about 14000 mmol/kg and preferably about 1200 mmol/kg carboxylic acid surface groups as determined by conductometric titration.

Additionally, MxG-CNF-CO₂Hs are formed having relatively high length to width ratios. Aspect ratios of about 190 to about 260 and preferably about 230 are obtained as determined by atomic force microscopy analysis using the procedure described herein. Stated in another manner, MxG-CNF-CO₂Hs have been obtained having a width of about 3.8±0.8 nm and length of about 880±300 nm.

Wide-angle X-ray scattering measurements show the crystallinity of MxG-CNF-CO₂H is generally about 68% to about 73%, and typically about 70% and determined by procedures described herein.

The MxG-CNF-CO₂Hs have been found to exhibit good dispersibility in water and polar organic solvents, such as DMF, especially as compared to corresponding cellulose nanocrystals obtained from MxG.

Composites Including MxG Cellulose Nanofibers

Composites are formed comprising MxG-CNF-CO₂Hs and other components such as polymers and/or various additives. Many different types of materials can be mixed with the MxG-CNF-CO₂H in order to form a composite composition. Various materials include, but are not limited to, one or more polymers, one or more liquids and one or more non-polymeric materials. The MxG-CNF-CO₂Hs can be utilized in many different applications including, but not limited to, paper, plastics, rubber, paints, coatings, adhesives and sealants.

Composite compositions can include any desired amount of MxG-CNF-CO₂H. In one non-limiting embodiment, the MxG-CNF-CO₂Hs are present in an amount from about 0.1 to about 40 parts, and preferably in an amount from about 1 to about 20 parts based on 100 total parts by weight of a polymer present in the composition.

Many different polymers or copolymers can be utilized in the composite compositions of the present invention including MxG-CNF-CO₂H such as, but not limited to, ethylene oxide, propylene oxide, copolymers of ethylene oxide and epichlorohdrin and/or other monomers; a vinyl aromatic (co)polymer such as polystyrene and styrene copolymers; polyolefin polymers or copolymers such as polyethylene and polypropylene; diene polymers and copolymers, such as cis-polybutadiene; polyacrylates and acrylate copolymers, such as methyl methacrylate; poly(vinyl acetate); poly(vinyl alcohol); polyamides; poly(urethanes) and polyester polymers or copolymers such as polycaprolactone, poly(ethylene terephthalate) or polylactate.

In one embodiment, a solution containing the desired (co)polymer(s) and MxG-CNF-CO₂Hs can be mixed as desired, such as in order to obtain a substantially homogenous mixture, and then the solution can be cast or otherwise placed into a desired form and dried in order to produce a finished composite. In one embodiment the solution can be dried in a vacuum oven wherein suitable pressures, temperatures and drying times will vary depending upon the system utilized.

Various additives as known to those of ordinary skill in the art can be added to the composition in any desired amounts.

EXAMPLES

The MxG-CNF-CO₂H was characterized by various techniques, including Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), wide-angle X-ray diffraction (WAXD), atomic force microscopy (AFM) and rheology of its dispersions in water. Furthermore, poly(vinyl acetate) (PVAc) nanocomposites were prepared and their mechanical properties were investigated to understand the reinforcing capability of these nanomaterials.

Materials and Methods

Materials

MxG was obtained from Aloterra Energy LLC, Conneaut, Ohio. Sodium chlorite, sodium bromide, sodium hypochlorite, 2,2,6,6-tetramethylpiperidinyloxy (TEMPO), ammonium persulfate (APS), and polyvinyl acetate (PVAc, molecular weight (M_(w)) 100,000 g/mol) were purchased from Sigma-Aldrich, poly-L-lysine from Ted Pell, glacial acetic acid, dimethylformamide (DMF), hydrochloric acid (HCl), and sodium hydroxide (NaOH) were purchased from Fisher Scientific.

Pretreatment of MxG Stalks.

Ground up MxG powder (4 g) was soaked in 80 mL of 2 wt. % sodium chlorite solution containing 0.5 mL of glacial acetic acid. The mixture was stirred at 70° C. for 2 hours, then the suspension was filtered and thoroughly washed with deionized water. The resulting slurry was oven-dried at 40° C. to yield about 3 g (75%) of a slightly off-white powder.

Preparation of MxG-CNF-CO₂H by Ammonium Persulfate (APS) Oxidation

The bleached MxG powder (1 g) was immersed into 100 mL of 1M APS solution. The oxidation process was carried out with continuous stirring at 90° C. for 16 hours. The suspension was cooled to room temperature and centrifuged at 8000 rpm for 8 min. The centrifugation step was repeated three times with deionized water, and 1 M NaOH was added until the pH of the suspension reached 8, before being dialyzed with deionized water for two days. The material in the dialysis tubing was adjusted to a concentration of 0.1% and subjected to an ultrasonic homogenizer at 20 kHz and 200 W output power for 2 min (13 mm probe tip diameter, Branson SFX550). The large agglomerates were removed from the suspension by centrifugation at 8,000 rpm for 5 min. The supernatant was lyophilized to yield 0.33 g of the separated cellulose nanofibers (MxG-CNF-CO₂H) as a white powder.

Preparation of MxG-CNC-CO₂H

MxG-CNCs were prepared according to the previously reported literature procedure.⁴⁹ Ground up MxG stalk (8 g) was soaked in 250 mL of 2 wt. % sodium hydroxide solution at room temperature for 24 h. The remaining solid was filtered and this process was repeated twice more but this time at 100° C. The resulting solid was then added to 180 mL of 2 wt. % sodium chlorite with 8 drops of glacial acetic acid and stirred at 70° C. for 2 h. The product was washed with deionized water and freeze dried before being hydrolyzed with 1 M HCl at 75° C. for 15 h (30 mL acid for 1 g of the freeze-dried material). The hydrolyzed MxG was washed and dialyzed in deionized water, before being oxidized via a TEMPO-mediated oxidation procedure. Specifically, hydrolyzed MxG (1.00 g) was dispersed via sonication in 150 mL deionized water before 0.123 g TEMPO, 1.23 g NaBr and 1.23 g NaClO was added. The reaction was stirred for 4.5 h at room temperature while the pH was kept at 10 with the addition of 1 M NaOH solution. The reaction was stopped by adjusting the pH to 6 with 5 M HCl, at which point methanol was added and the mixture centrifuged to collect the solid precipitate of carboxylic acid functionalized MxG-CNC (MxG-CNC-CO₂H). The MxG-CNC-CO₂H were then suspended in water and dialyzed against deionized water for two days before being freeze-dried.

Preparation of MxG-CNF-CO₂H/PVAc Nanocomposites

MxG-CNF-CO₂H was dispersed in DMF at a concentration of 2 mg/L by sonication, Polyvinyl acetate was also dissolved in DMF at a concentration of 2 mg/mL. The CNF/PVAc nanocomposites with 1%, 5% and 10% of CNF were prepared by mixing the desired amount of the CNF dispersion and PVAc solution. The mixtures were mixed homogenously by a vortex mixer and then cast onto a Teflon petri dish and placed in a vacuum oven at 60° C. for four days. The resulting films were compression molded between spacers in a Carver laboratory at 90° C. with a pressure of 5000 psi for 15 min to obtain nanocomposite films with about 200 μm thickness. The CNC/PVAc nanocomposite films were prepared by the same method as CNF/PVAc films.

Characterization

Conductometric Titrations

The amount of carboxylic acid groups on the MxG-CNF-CO₂H was determined through conductometric titrations with a Accumet XL benchtop pH/conductivity meter (Fisher Scientific). Briefly, 25 mg of CNF was dispersed in 80 mL DI water by sonication in a Branson CPX sonication bath. Then the pH of this dispersion was adjusted to 3 by adding 15 μL of concentrated HCl (33 wt. %). The titration was performed using 0.01 M NaOH solution until the pH reached ca. 11. The conductivity was plotted against the volume of consumed NaOH, the flat part of the curve on the titration graph was used to determine the carboxylic acid group content.

Scanning Electron Microscope (SEM)

The ground MxG and solid material after each treatment step were sputter-coated with 5 nm layer of Pd/Pt and then characterized using a Zeiss Merlin SEM at 5 kV acceleration voltage.

Atomic Force Microscopy (AFM)

The dimensions of MxG-CNF-CO₂H were measured by AFM using a Bruker Multimode 8 instrument equipped with a Nanoscope 5 controller. MxG-CNF-CO₂H was dispersed in water at a concentration of 0.01% (w/w), then a drop of this MxG-CNF-CO₂H dispersion was placed on a freshly cleaved mica surface that has been pretreated with poly-lysine, and after five minutes the excess dispersion was rinsed off with water. The images were acquired using ScanAsyst mode. The length and thickness of the CNFs were analyzed by using Gwyddion software.

Fourier Transform Infrared Spectroscopy (FTIR)

Samples were characterized via ATR-FTIR spectroscopy (Shimadzu). Solid samples were placed directly on the ATR crystal, and the pressure clamp turned to its slip-clutch limit to maximize pressure on the samples. The spectra of samples were averaged from 46 scans from 500 to 4000 cm⁻¹ with a resolution of 4 cm⁻¹.

Thermal Gravimetric Analysis (TGA)

TGA was carried out using a thermogravimetric analyzer (TA Instrument Discovery). Samples were heated from 30 to 650° C. under a nitrogen atmosphere at a heating rate of 10° C./min.

Wide-Angle X-Ray Scattering (WAXS)

X-ray diffraction patterns were recorded using a SAXSLAB GANESHA 300XL system with Cu Kα source (λ=0.154 nm) at a voltage of 40 kV and 40 mA power. Powder samples were tightly packed inside plastic washers and were held in place between two pieces of Kapton tape,

Rheology

The rheological properties of the MxG-CNC and MxG-CNF aqueous dispersions were analyzed with an ARES-G2 rheometer (TA Instrument, USA) using a cone-plate fixture (25 mm diameter, 1° angle) equipped with a solvent trap to prevent water evaporation. The dynamic shear properties were measured during a strain sweep at a frequency of 1 Hz and a constant temperature of 20° C. The storage modules G′ as well as loss modulus G″ were measured as a function of oscillation strain from 0.01% to 500%.

Dynamic Mechanical Analysis (DMA)

Dynamic mechanical analysis experiments were conducted in tension mode at a fixed frequency of 1 Hz at 0.1% strain using a RSA-G2 solids analyzer (TA Instruments). The samples were cut to a rectangular shape with about 4 mm in width and 25 mm in length. The temperature scan was performed between 20 to 90° C. at a heating rate of 3° C./min.

Results and Discussion

Isolation of nanocellulose from Miscanthus x. giganteus (MxG) using APS.

Initial procedures employed to isolate nanocellulose from ground MxG stalk followed the ammonium persulfate procedure described in the literature,²⁷ which reported that CNCs were isolated from biosource such as flax, wood and cotton. However, this protocol (1 g ground MxG stalk in 100 mL of 1 M APS solution at 90° C. for 16 hours) failed to produce CNCs from MxG, but instead resulted in high aspect ratio nanofibers (FIG. 10 ) in a very low yield (ca. 6%). While the formation of high aspect ratio nanocellulose is interesting, the low yield hindered any further studies. As such, studies were carried out to see if this MxG nanocellulose obtained with APS could be accessed in higher yield. It was hypothesized that if some of the lignin and hemicelluloses (ca. 50% of the mass) could be removed⁵⁰ before the APS treatment it would aid diffusion of the APS into the cellulose fibers and improve the yield of the nanocellulose. As such the MxG stalk (FIG. 1 a ) was ground into fine particles (FIG. 1 b ) using a blender and subjected to a bleaching step (2 wt. % sodium chlorite and acetic acid at 70° C.) to convert most of the lignin into soluble phenolic compounds.⁵¹ The resulting off-white colored solid (FIG. 1 c ) was obtained in ca. 75% overall yield from the starting MxG powder. This bleached MxG material was then subjected to APS oxidization at 90° C. for 16 hours to remove the remaining lignin and hemicelluloses and convert some of the cellulose hydroxyl groups into carboxylic acid groups.²⁷ Finally, the MxG nanocelluose (FIG. 1 d shows the freeze-dried MxG nanocelluose) was obtained after subjecting the suspension of APS treated MxG to ultra-sonication in an overall yield of 25% from the ground MxG stalk.

Characterization of the MxG Nanocellulose Obtained Using APS.

To understand the effect that each treatment has on the material, SEM was used to image the solid after each step. The starting MxG powder consists of large particles with length about 600 μm and width about 150 μm (FIG. 2 a ), after the bleaching treatment, the dimension of particles showed no obvious change (FIG. 2 b ). Upon treatment with APS, the MxG sample had a very different appearance showing higher aspect ratio fiber-like morphologies with significant reduction in particle width to about 15 μm (FIG. 2 c ). After sonication no features can be observed by SEM (FIG. 2 d ), consistent with destruction all of the micron-sized MxG fibers/aggregates. To examine this final product in more detail, AFM studies were conducted and revealed that it consists of long and flexible nanofibers with an average width of about 3.8±0.8 nm and average length of about 880±300 nm (FIG. 3 ). The width and length distribution histograms of MxG-CNF-CO₂H are shown in FIG. 11 . This is much longer than the MxG-CNC-CO₂H obtained using the previously reported base/bleach/acid/TEMPO oxidation protocol (length ca. 300 nm).⁴⁹ It is expected that during the APS treatment, carboxylic acid groups are formed in the less crystalline regions and the surface of the crystalline regions of the MxG cellulose,²⁷ which (as the carboxylic acid moieties are deprotonated under basic conditions) increases the repulsion force between the nanofibers, thus making it is easier to separate the microfibers into individual nanofibers during sonication. Conductometric titrations confirm that there are about 1.2 mmol/g (1,200 mmol/kg) of negative charges on these MxG-CNF-CO₂H.

To further understand the chemical structure of the MxG-CNF-CO₂Hs obtained by this APS procedure, the FTIR spectra of the MxG powder, bleached MxG and the MxG-CNF-CO₂Hs under basic and acidic conditions were compared to the MxG-CNC-CO₂H (FIG. 12 ) prepared using the literature procedure.⁴⁹ The spectra for these four samples all show the characteristic peaks of cellulose, including a broad OH stretching at 3400 cm⁻¹, the peaks at 2900 and 1030 cm⁻¹ are assigned to C—H stretching and CH₂—O—CH₂ stretching, respectively.⁵²⁻⁵³ The detailed partial spectra from 1900 cm⁻¹ to 1300 cm⁻¹ are shown in FIG. 4 . The peak at 1734 cm⁻¹ present in the MxG powder and bleached MxG have been previously assigned as the carbonyl stretching vibrations from hemicellulose.⁵⁴ The peaks at 1604, 1512, and 1462 cm⁻¹ are assigned to the aromatic vibrations which are typically observed from the aromatic skeleton of lignin.⁵⁵ The absence of these four absorption peaks (1734, 1604, 1512, 1462 cm⁻¹) in the MxG-CNF confirms the efficient removal of lignin and hemicellulose from the stalk of MxG after bleach treatment and APS oxidation. The broad peak at 1600 cm⁻¹ observed in the MxG-CNF-CO₂H sample is consistent with the presence of carboxylate moieties. This peak shifts to 1730 cm⁻¹ upon treatment with HCl consistent the presence of carboxylic acid moieties, further confirming the successful oxidation of hydroxyl groups to carboxylates on the MxG-CNF during the APS treatment,⁵⁶ It is worthwhile pointing out that the peak at 1645 cm⁻¹ (most obvious in the bleached sample) is due to OH bending of adsorbed water. While all samples were carefully dried, the adsorbed water is difficult to completely remove due to the strong cellulose-water interactions.⁵⁷

One of the key differences (other than size) between CNCs and CNFs is their degree of crystallinity. As such X-ray diffraction studies were carried out on ground MxG, bleached MxG, MxG-CNF-CO₂H as well as MxG-CNC-CO₂H for comparison (FIG. 5 ). As expected the XRD pattern of the ground MxG shows less defined crystalline peaks consistent with the relatively low fraction of crystalline cellulose in this material (MxG stalks contains about 50% of cellulose).⁴⁵ After the bleaching step, two broad peaks are observed at ca. 15.4° (2θ) and ca. 22.6° (2θ) consistent with cellulose I patterns, and the removal of most of the non-cellulosic components (such as lignin, hemicellulose). The APS treated sample shows similar peaks which correspond to the main crystalline region of cellulose, with the broad peak around 15.4° corresponding to the 1ī0 and 110 planes (which are not clearly resolved) and at the peak 22.6° corresponding to the 200 plane.⁵⁶

The crystallinity index (CI) of cellulose was defined according to the following equation:⁵⁹

$\begin{matrix} {{C.I.} = {\frac{I_{200} - I_{AM}}{I_{200}} \times 100\%}} & (1) \end{matrix}$

where I₂₀₀ is the intensity of the plane 200 reflection, I_(AM) is the intensity at 2θ=18°, corresponding to the minimum between the planes 200 and 110 in the diffractogram.⁶⁰ The degree of crystallinity was calculated using equation (1) from the height ratio between the intensity of the crystalline peak (I₂₀₀-I_(AM)) and the total intensity (I₂₀₀) after subtraction of the measured background intensity (using only Kapton tapes without any cellulose sample). The crystallinity index for the bleached MxG is about 61 after APS oxidation, the crystallinity index of the CNFs increases to 70%, suggesting that the APS treatment does remove some, but not all, of the noncrystalline regions of the cellulose during the oxidation process. For comparison, the crystallinity index of MxG-CNC-CO₂H measured the same way was 86%.

Having investigated the structure of the MxG-CNF-CO₂H the next step was to understand the properties of these nanomaterials. Initial studies focused on understanding their thermal properties. To this end ground MxG, bleached MxG and MxG-CNF-CO₂Hs were investigated by TGA and compared with MxG-CNC-CO₂Hs. The weight loss and the derivate weight loss curve plotted as a function of temperature are shown in FIGS. 6 (a) and (b), respectively. The initial weight loss for ground and bleached MxG is around 200° C. in a N₂ atmosphere and the maximum peak in the derivative weight loss occurs around 350° C. in both samples. Both ground MxG and bleached MxG have a shoulder at 290° C. and 275° C., respectively (FIG. 6 b ). MxG-CNF-CO₂Hs show two peaks in the TGA, around 270° C. and 330° C., the former is presumably a consequence of the thermal degradation of the sodium anhydroglucuronate units.⁶¹ The latter peak is lower than that of the native cellulose (350° C.), and has been assigned to easier degradation of the cellulose chains on account of the anhydroglucuronate units in the nanofibrils.⁶² Interestingly, the MxG-CNF-CO₂Hs and MxG-CNC-CO₂Hs show very different decomposition behavior. Both the MxG-CNF-CO₂Hs and MxG-CNC-CO₂Hs exhibit two decomposition peaks but the transitions in the MxG-CNC-CO₂Hs occur at lower temperatures (235° C. and 290° C.) than the MxG-CNF-CO₂Hs transitions (270° C. and 330° C.). Thus, the MxG-CNF-CO₂Hs made via this APS route have better thermal resistance than MxG-CNC-CO₂Hs. This might be slightly unexpected given the higher amorphous content of the MxG-CNFs but is consistent with the APS treatment being less harsh than the HCl hydrolysis/TEMPO-mediated oxidation process used to access the MxG-CNC-CO₂Hs. This would result in fewer defects on the CNF surface and/or reduced cellulose chain degradation. Another difference between MxG-CNF-CO₂Hs and MxG-CNC-CO₂Hs is that MxG-CNF-CO₂Hs have a lower char yield (ca. 10 wt. %), compared to the MxG-CNC-CO₂Hs (about 22% wt. % residue), presumably, at least in part, a consequence of the lower crystalline content of the CNFs.

Dispersibility of the MxG Nanocelluose:

The ability to disperse nanoparticles in a solvent is important for both functionalization as well as processing.⁶³ Thus, the dispersibility of the MxG-CNF-CO₂Hs was tested in two solvents commonly used for CNCs, water (a polar protic solvent) and dimethylformamide (DMF, a polar aprotic solvent) and compared with the dispersibility of the MxG-CNC-CO₂Hs. The samples were prepared at various concentrations (0.2, 0.4, 0.6, 0.8 and 1.0 wt. %) using an ultrasonicator. MxG-CNC-CO₂Hs disperses in water at these concentrations and forms transparent/translucent suspensions depending on the concentration (FIG. 7 a ). The MxG-CNF-CO₂Hs also disperse well in water forming homogeneous suspensions, but were less transparent (FIG. 7 b ). After one day no difference is observed with the MxG-CNC-CO₂Hs (FIG. 7 c ). However, the MxG-CNF-CO₂Hs form a gel at 0.6 wt. % and above (FIG. 7 d ). In DMF the MxG-CNC dispersions showed lower transparency than in water and started to settle after one day (FIG. 8 a and c), presumably due to the lower dielectric permittivity (ε) of DMF (which is 38) compared to water (ε=80). However, as with the aqueous dispersions, MxG-CNF-CO₂Hs DMF dispersions (FIG. 8 b ) were quite stable, and dispersions with 0.6 wt. % and above form a gel after one day (FIG. 8 d ). Furthermore, no settlement was observed in the MxG-CNF-CO₂Hs DMF dispersions over a period of a few months. Viscoelastic measurements (FIG. 13 ) showed that the G′ and G″ of MxG-CNF-CO₂Hs dispersion are much higher than that of the MxG-CNC-CO₂Hs dispersion at the same concentration, and for MxG-CNF-CO₂Hs dispersion at concentrations at 0.4 wt. % and above, G′ is higher than G″ in the linear viscoelastic region indicative of “gel-like” behavior,⁶⁴ while MxG-CNC-CO₂Hs dispersions only exhibited this type of behavior at 1.0 wt. %. This is presumably related to the higher aspect ratio of MxG-CNF-CO₂Hs (ca. 230) relative to the MxG-CNC-CO₂Hs (ca. 65) allowing the CNFs to entangle/percolate and form a gel network at the lower concentrations.

Comparison of the Mechanical Properties of MxG-CNF-CO₂H and MxG-CNC-CO₂H Nanocomposites.

One key application of cellulose nanomaterials is the reinforcement of polymer matrices. Previous research from numerous groups have shown that CNCs⁶⁵⁻⁶⁷ and CNFs,^(10, 68) isolated from various biosources, can be used as reinforcing fillers. Thus, it was of interest to see how these new MxG-CNF-CO₂Hs performed in this capacity. A series of dried MxG-CNF-CO₂H/PVAc composites with different weight percent of MxG-CNF-CO₂Hs (1, 5 and 10 wt. %) were prepared and compared to similar MxG-CNC-CO₂H/PVAc nanocomposite films prepared in a similar manner. Their thermomechanical properties were studied by Dynamic Mechanical Analysis (DMA) as shown in FIG. 9(a), As is expected the DMA temperature sweeps of the MxG-CNF-CO₂H and MxG-CNC-CO₂H composites show an increase in tensile storage modulus (particularly above T_(g)) with increasing nanocellulose content. The tensile storage modulus at 80° C. was used to compare the reinforcement capability of the nanocellulose since it is well above T_(g) of these composites (FIG. 9(b)). At 1 wt. % of MxG-CNF-CO₂H the tensile storage modulus of the composite at 80° C. is 4±0.5 MPa, and at 5 wt. % and 10 wt. % of MxG-CNF-CO₂H the storage modulus increases to 48±2 MPa and 106±8 MPa at 80° C., respectively. Interestingly, the MxG-CNF-CO₂H shows a significantly better reinforcement capability for PVAc than the MxG-CNC-CO₂H. For example, at 10 wt. % of nanocellulose filler content, the tensile storage modulus for MxG-CNC-CO₂H/PVAc composite is 16±1 MPa while for MxG-CNF-CO₂H/PVAc composite is 106±8 MPa. The difference in aspect ratio between these two nanocelluloses (65 for MxG-CNC-CO₂H and 230 for MxG-CNF-CO₂H), presumably contributes to the difference in reinforcement capability at the same content, with the larger aspect ratio fiber being able form a stronger percolation network.

Direct APS Oxidation of MxG Powder

The MxG powder (1 g) was directly mixed with 100 mL of 1M APS solution. The suspension was stirred at 90° C. for 16 hours, before being cooled to room temperature and centrifuged at 8000 rpm for 8 min. The centrifugation step was repeated three times with deionized water. After adjusting pH to 8 by adding 1 M NaOH, the suspension was dialyzed in deionized water for two days. The suspension was adjusted to a concentration of ca. 0.1% and treated with an ultrasonic homogenizer at 20 kHz and 200 W output power for 2 min (13 mm probe tip diameter, Branson SFX550). Non-dispersed particles were removed from the suspension by centrifugation at 8000 rpm for 5 min. The supernatant was lyophilized to yield a white powder (ca. 0.06 g). The nanofibers obtained from this treatment have a width of ca. 4.0±0.7 nm and a length of ca. 960±270 nm by AFM measurement (FIG. 10 ).

Modulus

Five different concentrations (0.2, 0.4, 0.6, 0.8 and 1.0 wt. %) of both the MxG-CNC-CO₂H and MxG-CNF-CO₂H samples were analyzed under oscillatory shear and the results are shown in FIG. S4 . For all the samples tested, a linear viscoelastic region was observed at lower oscillatory strains, where the modulus is independent of applied shear strain, and the dynamic modulus increased as the concentration increased.¹ However, the G′ and G″ of MxG-CNF-CO₂H dispersion are much higher than that of the MxG-CNC-CO₂H dispersion at the same concentration, which indicates a the formation of a stronger network in water is formed by CNFs, presumably a result of their longer length and higher aspect ratio. For MxG-CNF-CO₂H dispersions at concentrations higher than 0.4%, the G′ are higher than G″ in the linear region which indicate a “gel-like” behavior,² while MxG-CNC-CO₂H dispersions only exhibited this type of behavior at 1.0%. This result also confirmed that the MxG-CNF-CO₂H are able to form gel networks even at low concentrations.

Synopsis

A new process was developed for the isolation of carboxylic acid functionalized cellulose nanofibers (MxG-CNF-CO₂Hs) from the sustainable grass hybrid Miscanthus x. giganteus. MxG-CNF-CO₂Hs with, in certain embodiments, a relatively high (compared to MxG-CNCs) aspect ratio of 230 were successfully obtained after a bleach treatment, followed by an APS oxidation. The overall yield of this process is about 25% by weight from starting the MxG stalks. The crystallinity of these MxG-CNF-CO₂Hs is about 70%, and the content of its surface carboxylic acid groups is about 1.2 mmol/g. Similar to the related MxG-CNC-CO₂Hs, the MxG-CNF-CO₂Hs have good dispersibility in water, and show better dispersibility in DMF. The MxG-CNF-CO₂Hs were also shown to form gels at lower concentrations than observed with the MxG-CNC-CO₂Hs. PVAc nanocomposites were made with MxG-CNF-CO₂Hs and MxG-CNC-CO₂Hs at different nanofiller contents. MxG-CNF-CO₂Hs show a significantly higher reinforcement effect than MxG-CNC-CO₂Hs. As such using bleach plus APS, a cheap and low toxicity oxidant, provides a relatively simple method for preparation of cellulose nanofibers from Miscanthus x. giganteus, These MxG-CNF-CO₂Hs exhibit good dispersibility in aqueous and polar organic solvents and show good polymer reinforcement capabilities.

For the avoidance of doubt, the compositions, article and methods of the present invention encompass all possible combinations of the components, including various ranges of said components, disclosed herein. It is further noted that the term ‘comprising’ does not exclude the presence of other elements. However, it is also to be understood that a description of a product comprising certain components also discloses a product consisting of these components. Similarly, it is also to be understood that a description on a process comprising certain steps also discloses a process consisting of these steps.

While in accordance with the patent statutes the best mode and preferred embodiment have been set forth, the scope of the invention is not limited thereto, but rather by the scope of the attached claims.

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What is claimed is:
 1. A process for preparing cellulose nanofibers from Miscanthus x. giganteus (MxG), comprising the steps of: performing a bleaching step on a quantity of the MxG; and thereafter subjecting a solid material obtained from the bleaching step to ammonium persulfate oxidation and obtaining carboxylic acid functionalized cellulose nanofibers, MxG-CNF-CO₂H.
 2. The process according to claim 1, further including the step of obtaining the quantity of the MxG and reducing a particle size thereof to a smaller average particle size.
 3. The process according to claim 2, wherein the reduced average particle size is less than 500 μm.
 4. The process according to claim 1, wherein the bleaching step comprises contacting the MxG with a bleaching solution comprising an oxidizing agent and one or more of an acid and an acid liberating agent.
 5. The process according to claim 4, wherein the oxidizing agent comprises one or more of sodium chlorite, sodium hypochlorite, calcium hypochlorite and hydrogen peroxide.
 6. The process according to claim 5, wherein the concentration of the oxidizing agent ranges from about 2 wt. % to about 4 wt. % based on the total weight of the bleaching solution.
 7. The process according to claim 4, wherein the pH of the bleaching solution is between about 4 and about
 6. 8. The process according to claim 6, wherein the bleaching step is performed at a temperature between 60° C. and 80° C. for an amount of time sufficient to convert lignins present in the MxG.
 9. The process according to claim 1, wherein after the bleaching step has been performed, the MxG-containing bleaching solution is filtered, washed and dried prior to subjecting the solid material obtained from the bleaching step to ammonium persulfate oxidation.
 10. The process according to claim 1, wherein the ammonium persulfate oxidation step includes contacting the solid material obtained from the bleaching step with an ammonium persulfate solution having a concentration from about 0.5 M to about 2.0 M.
 11. The process according to claim 10, wherein the ammonium persulfate solution containing the solid material is mixed fora suitable period of time in order to oxidize the solid material to form the MxG-CNF-CO₂H.
 12. The process according to claim 11, wherein the mixing is performed at a temperature between about 50° C. to about 90° C.
 13. The process according to claim 12, wherein the oxidation process is performed for about 8 hours to about 24 hours. 